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The self-confessed "edgy" card retailer Scribbler has 35 stores across the UK - all of which display a "PG" sticker in their windows.
The fashion and lifestyle chain Joy - based mainly in the south-east of England - also carries PG signs.
Some cards are shocking, rude and downright offensive, yet there is "huge demand" for profanities, according to retailers.
"Customers love it," said Eliot James, a card buyer for Scribbler. "It's normally the ruder end of humour which are the bestsellers."
And the offending greetings are not, he said, limited to a certain demographic or age group.
"It's really quite widespread," he added.
Warning: Some readers may find the images below offensive
As well as having PG signs at the front of their shops, Scribbler says cards featuring swear words should only be found on the top two shelves.
Joy said offending cards are not kept at till level - but above children's heights.
Sharon Little from the Greeting Card Association said there was "huge demand" for rude cards.
"They've always been popular," she said.
"You send a card to show that you care about someone - it's sharing a sense of humour.
"But you have to know someone really well to choose a card like that for them."
But not everyone is happy.
Earlier this year, Paperchase - which stocks more than 4,000 cards - was forced to pull a line of sexually explicit Valentine's cards after some shoppers complained.
The cards appeared behind a sign stating "rude cards here - you have been warned!"
But that wasn't enough to satisfy all customers - especially those with young children.
"Adult cards can undoubtedly polarise customer opinion and this Valentine's Day we took the controversial decision to remove some of the more risquÃ© cards from our stores so that we didn't cause unnecessary offence to customers," a Paperchase spokeswoman said.
"While this decision proved popular with some, we also had many customers who were disappointed."
Paperchase said their adult cards are only sold in select stores where teams are asked "to position them on the top shelf, out of sight of children and with a sign".
But the cards certainly don't seem to be putting a dent in the tradition of card sending.
In fact, the British public are buying more cards than ever before - with the UK market worth an estimated Â£1.7 bn.
So if you're feeling adventurous this Mother's Day, perhaps go for a more edgier take on the classic. Just make sure you inherited your sense of humour from your mother.
They are usually seen on the covers of films or computer games - but some of the UK's card shops have started carrying parental guidance signs.